1. Field of the invention
This invention, relating to iron-based permanent magnets and alloy powders for iron-based bonded magnets and their fabrication, used for obtaining suitable iron-based bonded magnets for all kinds of motors, actuators and magnetic circuits for magnetic sensors, as well as magnetic rolls and speakers, regards iron-based permanent magnets and their fabrication which yield isotropic iron-based bonded magnets having a residual magnetic flux density Br greater than 5 kG unobtainable from hard ferrite magnets. These are produced by quenching an (Fe,Co)xe2x80x94Crxe2x80x94Bxe2x80x94R molten alloy or a (Fe,Co)xe2x80x94Crxe2x80x94Bxe2x80x94Rxe2x80x94M (M=Al,Si,S,Ni,Cu,Zn,Ga,Ag,Pt, Au,Pb) molten alloy, with small amounts of rare earth elements added, using either, a melt-quenching process utilizing a rotating roll, splat, quenching, a gas atomizing method, or a combination of these methods, to obtain an essentially amorphous structure or a structure containing small amounts of microcrystals dispersed within an amorphous matrix, to yield an iron-based permanent magnet consisting of microcrystal clusters where both a soft magnetic phase consisting of a ferromagnetic alloy whose main components are xcex1-Fe and iron-based phases, and a hard magnetic phase having a Nd2Fe14B-type crystal structure coexist after applying a particular heat treatment. This is then ground to obtain an alloy powder for bonded magnets, which is then combined with a resin.
2. Description of the Prior Art
Permanent magnets used in stepping motors, power motors and actuators utilized in home electronic goods and electric goods in general are mainly limited to hard ferrites, which have various problems such as, demagnetization at low temperatures with the fall of iHc, the ease of formation of defects, cracks and a lowering of mechanical strength due to the quality of the a ceramic material, and the difficulty to fabricate complicated forms. These days, along with the miniaturization of home electronics and OA equipment, small, light-weight magnetic materials to be used in these products are being sought. As for motor vehicles, as much effort is being made towards saving money and resources by making vehicles light-weighted, even more small, light-weight electrical components for motor vehicles are being sought.
As such, efforts are being made to make the efficiency versus weight ratio of magnetic materials as large as possible and, for example, permanent magnets with a residual flux density Br in the range 5xcx9c7 kG are thought to be most suitable.
In conventional motors, for Br to be above 8 kG, it is necessary to increase the cross section of the iron plate of the rotor or stator which forms the magnetic path, introducing an associated increase in weight. Further, with the miniaturization of magnets used in magnetic rolls and speakers, an increase in Br is being sought as present hard ferrite magnets cannot give more than 5 kG Br.
For example, for a Ndxe2x80x94Fexe2x80x94B-type bonded magnet to satisfy such magnetic characteristics, 10xcx9c15 at % of Nd needs to be included making their cost incredibly high compared to hard ferrite magnets. Production of Nd requires many metal separation and reduction processes which in turn needs large scale equipment. As well as this, for 90% magnetization, a magnetic field of close to 20 kOe is required and there are problems with the magnetization characteristics such as being unable to achieve complicated multipole magnetization such that the pitch between the magnetic poles is less than 1.6 mm.
At present, there are no permanent magnet materials with magnetization characteristics such that Br is 5xcx9c7 kG which can be mass produced cheaply.
Recently, a Ndxe2x80x94Fexe2x80x94B-type magnet has been proposed whose main component is an Fe3B-type compound with a composition close to Nd4Fe77B19 (at %) (R. Coehoorn et al., J. De Phys., C8, 1988, 669xcx9c670). This permanent magnet has a semi-stable structure with a crystal cluster structure in which a soft magnetic Fe3B phase and a hard magnetic Nd2Fe14B phase coexist. However, it is insufficient as a rare earth magnetic material with a low iHc in the range 2 kOexcx9c3 kOe, and is unsuitable for industrial use.
Much research is being published, however, on adding additional elements to magnetic materials with Fe3B-type compounds as their main phase and creating multi-component systems, with the aim to improve their functionality. One such example is to add rare earth elements other than Nd, such as Dy and Tb, which should improve iHc but, apart from the problem of rising material costs from the addition of expensive elements, there is also the problem that the magnetic moment of the added rare earth elements combines antiparallel to the magnetic moment of Nd or Fe, leading to a degradation of the magnetic field and the squareness of the demagnetization curve (R. Coehoorn, J. Magn. Magn. Mat., 83 (1990) 228xcx9c230).
In other work (Shen Bao-gen et al., J. Magn. Magn. Mat., 89 (1991) 335xcx9c340), the temperature dependence of iHc was improved by raising the Curie temperature by replacing some Fe with Co, but this also caused a fall in Br on the addition of Co.
In each case, for Ndxe2x80x94Fexe2x80x94B-type magnets whose main phase is an Fe3B-type compound, it is possible to create hard magnetic materials with a heat treatment after amorphizing by quenching, but their iHc is low and the cost performance of using them instead of hard ferrite magnets is unfavourable. This incapability of providing a high-enough iHc is caused by a large grain size of the soft magnetic phase, typically 50 nm, which is not small enough to effectively prevent magnetization rotation in the soft magnetic phase from occurring under of a demagnetization field.
The purpose of this invention is to present (Fe,Co)xe2x80x94Crxe2x80x94Bxe2x80x94R-type permanent magnets or (Fe,Co)xe2x80x94Crxe2x80x94Bxe2x80x94Rxe2x80x94M (M=Al,Si,S,Ni, Cu,Zn,Ga,Ag,Pt,Au,Pb)-type permanent magnets and their iron-based bonded magnets which can be cheaply produced by stable industrial processes, where these magnets contain few rare earth elements but have a residual magnetic flux density Br above 5 kG matching hard ferrite magnets in cost performance.
Further, in order to provide a cheap, stable industrial process for bonded magnets with a residual magnetic flux density Br above 5 kG, this invention provides iron-based permanent magnet alloy powders for iron-based permanent magnets suitable for bonded magnets and iron-based bonded magnets, and their fabrication.
The inventors, as a result of various investigations into possible fabrication methods by stable industrial processes to increase the iHc of iron-based permanent magnetic materials with low rare earth content and containing both soft and hard magnetic phases, have, quenched an molten alloy with a particular composition containing few rare earth elements, in which Cr, or Cr and M ((M=Al,Si,S,Ni,Cu,Zn,Ga,Ag,Pt, Au,Pb), were simultaneously added to an iron-based alloy partially substituted with Co, by melt quenching using a rotating roll, by splat quenching, by gas atomizing or by a combination of these methods, and, after obtaining an essentially amorphous structure or a structure containing small amounts of microcrystals dispersed within an amorphous matrix, have obtained by a particular heat treatment at a particular heating rate, an iron-based permanent magnet in ribbon or flake form consisting of microcrystal clusters in which soft magnetic phases containing a ferromagnetic alloy, whose main components are xcex1-Fe and a interemetallic compound with iron as its main phase, and hard magnetic phases, having a Nd2Fe14B-type crystal structure, coexist. By grinding and forming an alloy powder of this material to form a bonded magnet, they have completed this invention by obtaining an iron-based bonded magnet having a residual magnetic flux density Br above 5 kG, unobtainable with a hard ferrite magnet.
Thus, both soft magnetic phases consisting of a ferromagnetic alloy whose main components are xcex1-Fe and iron-based phase, and hard magnetic phases having a Nd2Fe14B-type crystal structure will coexist within the same powder particles, and so, for mean crystal particle sizes of each constituent phase in the range of 1 nmxcx9c30 nm, an intrinsic coercive force above the realistically required 5 kOe is apparent and, by molding magnetic powder having a particle size of 3 xcexcmxcx9c500 xcexcm into the required forms using a resin, they can obtain permanent magnets in a usable form.
For this invention, after an (Fe,Co)xe2x80x94Crxe2x80x94Bxe2x80x94R-type or (Fe,Co)xe2x80x94Crxe2x80x94Bxe2x80x94Rxe2x80x94M (M=Al,Si,S,Ni, Cu,Zn,Ga,Ag,Pt, Au,Pb)-type molten alloy with a particular composition containing few rare earth elements is quenched by melt quenching using a rotating roll, by splat quenching, by gas atomizing or by a combination of these methods, and, after an essentially amorphous structure or a structure containing small amounts of microcrystals dispersed within an amorphous matrix has formed, it is crystallized by further heat treatment, the crystallization heat treatment being to raise the temperature at a rate of 10xc2x0 C. per minutexcx9c50xc2x0 C. per second from the temperature at the start of crystallization to a treatment temperature of 600xc2x0 C.xcx9c700xc2x0 C., and so obtain microcrystal clusters where the mean crystal size of each component phase is in the range 1 nmxcx9c30 nm and where both a soft magnetic phase consisting of a ferromagnetic alloy whose main components are xcex1-Fe and a alloy with iron as its mainphase, and a hard magnetic phase having a Nd2Fe14B-type crystal structure coexist within the same powder particles. We can obtain iron-based permanent magnets in ribbon or flake form having the following magnetic characteristics.
For the (Fexe2x80x94Co)Crxe2x80x94Bxe2x80x94R-type,
iHcxe2x89xa75 kOe, Brxe2x89xa78.0 kG, (BH)maxxe2x89xa710 MGOe and in the case of the (Fexe2x80x94Co)Crxe2x80x94Bxe2x80x94Rxe2x80x94M-type,
iHcxe2x89xa75 kOe, Brxe2x89xa78.2 kG, (BH)maxxe2x89xa710.5 MGOe.
Further, we can obtain iron-based permanent magnet alloy powders suitable for bonded magnets having a residual magnetic flux density Br above 5 kG by grinding this material as required to a mean powder particle size of 3 xcexcmxcx9c500 xcexcm and so can obtain iron-based permanent magnet alloy powders having the following magnetic characteristics.
For the (Fexe2x80x94Co)Crxe2x80x94Bxe2x80x94R-type powder,
iHcxe2x89xa75 kOe, Brxe2x89xa77.0 kG, (BH)maxxe2x89xa78 MGOe and in the case of the (Fexe2x80x94Co)Crxe2x80x94Bxe2x80x94Rxe2x80x94M-type powder,
iHcxe2x89xa75 kOe, Brxe2x89xa77.2 kG, (BH)maxxe2x89xa78.4 MGOe.
Finally, by combining this powder with a resin, we can obtain a bonded magnet with the following magnetic characteristics.
iHcxe2x89xa75 kOe, Brxe2x89xa75.5 kG, (BH)maxxe2x89xa76 MGOe.